Journal repeatedly rolls over when tree handle map gets too big

[Peter Beaman]

We think this is true as a result of design review - to date it has not been reported and we have not yet written a bug report.

The JournalManager keeps a map of handle -> TreeDescriptor (and also a different map of handle -> Volume). Because aTreeDescriptor is a lightweight object that does not actually have a reference to the Tree, we don't bother to remove it when a tree is removed. This means obsolete handle->TreeDescriptor pairs are written to the start of every journal file, normally without any consequence.

However, creating and removing an extremely large number of trees could load up the tree handle map to a size the would cause JournalCopier to roll the journal on every cycle. This could create a very large number of journal files, all of which would be deleted within a fairly short period of time.

As part of general cleanup of tree management we should remove the map entries for deleted journals and closed volumes.